feat:add job-class kwarg to rqworker

[gabriels1234]

Fixes #159

[gabriels1234]

@cunla I have added both a test for when --job-class is used and for when it's not used, ensuring proper behavior for both current django-tasks-scheduler users and also for those coming from django-rq-scheduler.
Would you like me to follow a different approach? like allowing any Job type, as a responsibility of the user?
In my case, I updated my CustomJob to inherit from JobExecution instead of rq.job.Job.

[gabriels1234]

@cunla please let me know if I can be of help.
Thanks!!